While recently visiting various sites to compare the I and II versions of this lens, I noticed that each one has its own sweet spot for the least distortion. If choosing between the two, I would, likely, try to determine what focal lengths were most important to me, and go with the lens with less distortion in that range.

I cannot remember the sweet spots of either lens, largely because I settled upon the 16-35mm 2.8L II as my likely first acquisition among L zooms; not because I dislike either 24-70mm, but because I decided to prioritize the wider and longer L zooms.